The present invention is generally directed to distributed data processing environments. More particularly, the present invention is directed to methods and systems for synchronizing global resources in an efficient manner. Even more particularly, the present invention is directed to the use of local and aggregate syndromes as a mechanism for maintaining consistent, synchronized global resources.
In distributed data processing environments, distributed applications often require information which includes information about peer processes or nodes. This information includes such things as globalized resource or configuration data regarding distributed resources or distributed configuration data. In other words, each peer process has its own set of resource (that is, it has one of the distributed resources), and also uses collected resources belonging to all peer processes (that is, uses globalized resources).
One of the solutions to the problem of global resource synchronization (that is, global resource consistency in the temporal domain) in a distributed data processing environment is the use of a central data repository to store copies of the resources so that every peer process is able to access, from one location, a globalized view of the resources. This method provides an easy way to globalize the distributed resources by simply updating the single central repository without introducing any concern for synchronization of the global resource between peer nodes. However, this method, with its central repository could easily become not only a single point of failure but could also easily become a bottleneck which severely and negatively impacts system performance. Without some form of central repository, coordination or synchronization, peer processes work incorrect or even fail. This is because, in this approach, globalization is carried out through a single location: every peer process is trying to access this location and thereby causes a performance degradation. Accordingly, the synchronization methods proposed herein seek to avoid such problems.
Another solution to the problem of synchronization of globalized distributed resources is the replication of the global resources on every peer node. In this approach, there is the advantage that there is no single point of the failure or performance bottleneck, because every peer node keeps a replicated copy of the globalized resource. However, there are still several issues and areas of concern that occur with resource replication as a solution method. For example, there is the issue of how one can efficiently distribute the resources. Additionally, assuming that one starts with a consistent set of distributed resources, there is also the issue of how to efficiently synchronize the replicated copy of the globalized resource as changes occur in the distributed environment. These concerns are exacerbated when the distributed resources are determined for the first time, as for example when a new node joins the peer computing environment and when the distributed resources need to be resynchronized whenever the one of distributed resources is changed or when one of the peer nodes either fails or leaves the established distributed environment.
Unfortunately, it is not only difficult to notify all of the peer processes, in a synchronized manner, concerning changes made in the distributed resources (for example, changes in their content, as opposed to other resource attributes such as time of last local access) so as to preclude the use of different resource contents for the globalized resource amongst the peer processes (or nodes), but it is also difficult to optimize the globalization of the distributed resources. That is to say, it is difficult to provide updated information to a plurality of peer applications running in a distributed system, particularly if the information needed is voluminous. Additionally, if a few peer processes miss the notification of the changes or receive the notifications asynchronously, the overall coordination between peer processes may fail or may cause unexpected and undesired results. The negative impact of the inconsistency problem is even greater under the following situations:                If two nodes have two different views of the globalized resource, the overall coordination between peer processes may fail, and in many cases, the distributed peer processes may produce unexpected effects.        The globalization process for synchronizing distributed resources between the nodes may introduce a severe performance hit, particularly at startup time when all of the nodes are involved.        If there is no automatic mechanism for the peer processes to maintain the globalized resource, there is also always a chance for inconsistent use of the globalized resource between the peer processes. As an example, it may happen that one of the peer processes does not receive the notification of the change in the distributed resource.        If the distributed resources are changed while a peer process is temporarily isolated, it is not easy for the peer process to get the current level of the globalized resource when isolation ceases.        
The consistency problem between peer processes has been dealt with to limited degrees in other systems, see “Group Communication in the Amoeba Distributed Operating System” by M. F. Kassshoek and A. S. Tanenbaum, Proceedings of the IEEE 11th International Conf. on Distributed Computing Systems, pp. 222–230, and “Design Alternatives for Process Group Membership and Multicast” by K. P. Birman, R. Cooper and B Gleeson, pp 1–31, 1991. The contribution to this art added by the present invention is the consistent synchronization and efficient globalization of the distributed resources provided, and the “ease of use” in identifying specific semantics related to the consistent and automatic propagation of the notifications between peer processes. This allows an application program to use the resource in a consistent manner at all times.
Therefore, it is important that applications have a mechanism to notify all peer processes concerning changes made to distributed resources in a synchronized manner as well as having a mechanism for the automatic propagation of the changes between peer processes. Applications which execute in an environment with tight consistency requirements should therefore be able to benefit from being provided with an infrastructure that not only provides synchronization but which also provides propagation of the changes in distributed resources. However, it is very difficult to create such infrastructure; if all of the applications are forced to do it on their own, each one runs the risks of repeating the same errors, and they may still lose consistency with each other. Accordingly, the present invention provides an infrastructure which solves these problems in an efficient manner which is available to all applications running in a distributed data processing environment.